In the well-known GSM (Global System for Mobile communications) there has been specified the so called FACCH or Fast Associated Control CHannel which is used for example to indicate the call establishment progress, to authenticate a subscriber or to command a handover. It is common to these “fast” signalling needs that all delays should be minimized and the conveyed information should be received over the radio interface as faultlessly as possible. As a background to the present invention we will shortly describe the known properties of a FACCH. A detailed description is found for example in the technical specifications number GSM 05.01, GSM 05.02 and GSM 05.03 published by the European Tele-communications Standards Institute.
The FACCH actually involves the use of the burst structure on a regular TCH or Traffic CHannel to temporarily convey “fast” signalling information instead of user data. The transmitting device packs the desired fast signalling information into an FACCH frame which, after channel coding, consists of 456 bits. At the interleaving stage the FACCH frame is split into 8 groups of 57 bits. The bits of the first group are transmitted in the even bit positions of a certain Nth transmission burst and the bits of the next groups go to the even bit positions of the next transmission bursts until the bits of the fourth group are transmitted in the even bit positions of the (N+3)th transmission burst. The bits of groups 5 to 8 are then transmitted in the odd bit positions of the transmission bursts N+4 to N+7 respectively. Certain stealing flags (i.e. indicator bits) are used within each transmission burst to indicate whether the even (or odd) bit positions of that particular transmission burst contain user data or fast signalling information. In other words the FACCH frame will be conveyed to the receiving device by using every second bit position in 8 consecutive transmission bursts of a certain traffic channel. If there is only one FACCH frame to be transmitted, all the other bit positions are used to convey user data.
As an example of the extensions to and developments over the existing cellular radio networks we will describe the proposed Enhanced Circuit Switched Data or ECSD arrangement which is currently being specified as a part of the Enhanced Data rates for GSM Evolution or EDGE programme. ECSD is based on enhancing the effective user data rates over the radio interface by employing 8-level Phase Shift Keying (8-PSK) as an alternative to the Gaussian Minimum Shift Keying or GMSK modulation method of GSM. A straightforward solution for implementing the fast signalling channels within ECSD would be to copy the above described method as closely as possible. In other words the information contents of an FACCH frame would be distributed selectively to the even and odd symbol positions in the 8-PSK modulated transmission bursts, and stealing flag symbols would be used to indicate the nature of the contents of each transmission burst.
However, one must note that 8-PSK as a modulation method requires a higher bit energy over noise density ratio (commonly referred to as Eb/N0) than GMSK to achieve a certain required level of faultlessness in the received information. It is therefore expected that simply adopting the GSM FACCH mechanisms in ECSD would lead to inadequate performance for the FACCH.
A proposed solution is to otherwise adopt the GSM practice but to use different detection metrics for the user data and fast signalling information, i.e. to effectively employ binary modulation like GMSK or BPSK (Binary Phase Shift Keying) for the FACCH symbols. This solution is unattractive from the receiver designer's point of view, because it would require a receiver to be able to detect every other symbol in a symbol sequence with a different detection algorithm. Another solution is to use a completely different physical channel with different channel specifications for conveying the fast signalling information, but this alternative tends to lead into complicated hardware structures.